The overall objective of ths study is to further the understanding of cardiac autorhythmicity by investigating electrogenesis in single cardiac cells under a number of environmental stress conditions. Of particular interest is the role of calcium in this process. Several analytical and experimental techniques are being employed including direct measurement of membrane potential, phase plane analysis, and computer analysis of experimental results. Changes in hydrostatic pressure are being used as an additional environmental stress with the hope of gaining further insight into the equilibria and kinetic variables governing membrane excitability. Finally, an experimental model comprised of spontaneously beating bat wing veins is being developed to aid in our investigations of pacemaker function.